Aqueous fluids that have been thickened, or viscosified, are useful in many industries, for example in the oilfield. Oilfield operations that use such fluids include, for example, enhanced oil recovery and well stimulation, for example hydraulic fracturing, gravel packing, and the combination of these two called frac-n-pack. Such fluids are commonly generated by incorporating, or forming, gels in them, for example viscoelastic surfactant gels or polymer gels. Polymer gels are typically formed by dissolving or hydrating a suitable polymer in water. Often, these aqueous fluids are further thickened or viscosified by crosslinking the polymers, for example with organic or metal crosslinkers. Typical metal crosslinkers include boron, titanium, and zirconium. Zirconium crosslinkers have been described, for example, in U.S. Pat. Nos. 5,614,475; 5,972,850; 5,950,729; 5,697,555; 4,799,550; 5,697,444 and 6,737,386.
For use in stimulation, valuable properties for fluids containing crosslinked polymer gels include low cost, simplicity of preparation, the ability to delay the gellation for a predetermined time (to minimize hydraulic horsepower required to pump the fluid), and thermal stability. In practice, moving toward those requirements with zirconium crosslinkers typically has meant that the crosslinking system had one or more of a known group of ligands on the zirconium (for example triethanolamine and/or lactate) in order to delay the crosslinking, and a pH modifier in order to increase the stability. Incorporation of such ligands increases the cost of the zirconium compounds. The amount (weight) of these ligands necessary for delay limits the zirconium concentration that can be provided in a crosslinker concentrate, and this plus potential competition for the zirconium between these ligands and pH modifier components means that pH modifiers are typically provided separately. Sometimes additional delay agents are also needed. This means that at least two additive streams are necessary just for the crosslinking and that the volume of crosslinker additive that must be used is large. These factors add to the complexity and cost of the operation.
There is a need for a crosslinking system that provides a high concentration of zirconium and a pH modifier in a single concentrate at reduced cost.